Custom post type + Dynamic slug = 404

You have probably to create a filter function that can handle this situation. Something like this:

/**
 * @param string $postlink
 * @param string $language
 * @return string
 */
function my_msls_options_get_permalink( $url, $language ) {
    if ( 'fr_FR' == $language ) {
        $url = str_replace( '/car/', '/voiture/', $url );
    }
    return $url;
}
add_filter( 'msls_options_get_permalink', 'my_msls_options_get_permalink', 10, 2 );